The term 'desktop lyrics' is just a casual expression, I have explained it in words with pictures,please don't mind it.
As for your question, for instance, when I'm listening to some non-native sounds, I need to rely on lyrics to understand their meaning. If there aren't desktop lyrics available, an awkward situation arises that I have to keep staring at this software. However, sometimes its two sentences can be quite distant,or when there are sentences that I can understand on my own,making the time in between very meaningless. If there were desktop lyrics, I could do more of what I want, just taking a glance while it reads the lyrics.
Of course, its benefits are definitely not limited to just this one,other people's needs will be different.